As top US government lawyer defends targeting citizens deemed an enemy of the state we ask if the action is justifiable.

It is one of the most controversial actions the US government can take: targetting one of its own citizens with lethal force.

At least three Americans have been killed overseas in the unprecedented drones programme under Barack Obama, the US president.

And Eric Holder, the US Attorney-General, has been laying out the most detailed legal justification for such action involving citizens deemed a threat to the nation, saying the US will continue to strike overseas if a nation is "unable or unwilling".

"What we are sanctioning here is murder. The president has no authority in Yemen… his power extends to the US, its territories and its possessions. But to authorise a civilian agency, the CIA, to go to Yemen with drones, fire a missile and kill American citizens is just breathtaking to most of us."

- Morris Davis, a former chief prosecutor at Guantanamo Bay

Holder said the US' legal authority is not limited to the battlefield and that action would continue be taken on foreign soil to deal with threats.

"We are at war with a stateless enemy, prone to shifting operations from country to country... But the use of force in foreign territory would be consistent with these international legal principles if conducted, for example, with the consent of the nation involved – or after a determination that the nation is unable or unwilling to deal effectively with a threat to the United States."

The most high-profile killing is that of Anwar al-Awlaki, the US-born cleric who was killed in a drone strike in Yemen in 2011.

Prosecutors say al-Awlaki instructed the so-called underwear bomber who tried to blow up a plane flying over Detroit in 2009 and was a leader of al-Qaeda in the Arabian Peninsula.

Critics argue the cleric was never charged with a crime and never had his day in court, raising questions about whether he got due process under the US constitution.

A number of lawsuits have been filed trying to force the US government to release its precise legal justification.

"I think al-Awlaki himself a fair target and this was an appropriate and legal strike. The two other Americans killed in the process of these raids are far more problematic and raise very difficult legal questions. …This is an extraordinary exercise of power and it's vital for the American people to know where the line is [drawn]."

- PJ Crowley, a former US State Department spokeperson

The American Civil Liberties Union has accused the administration of "claiming the authority to kill any American citizen whom the president deems to be an enemy of the state".

It is very difficult to confirm how many people have been killed in US drone attacks.

The London Bureau of Investigative Journalism has been collating the number of casualties in Afghanistan and Pakistan since Obama took office.

According to their research including credible eyewitness accounts, 535 civilians died in drone attacks in that period. Of those, 60 are reported to be children and around 20 mourners attending funerals. The report also said Obama has ordered 264 drone attacks, averaging one every four days.

The US administration has made a point of not speaking publicly about its drones programme.

In terms of Obama's policy of using drones, he is under little pressure to change his position judging by a poll this year.

So how convincing is the US government's legal argument for targetted killings based on an "imminent threat"? Are civil rights groups right to be concerned about the parameters of US authority far from any battlefield without judicial review of public scrutiny?

Joining a discussion on this with presenter Anand Naidoo on Inside Story Americas are guests: Jameel Jaffer, of the American Civil Liberties Union; PJ Crowley, a former US State Department spokeperson; and Morris Davis, a law professor at Howard University, and the former chief prosecutor at Guantanamo Bay between 2005 and 2007.

"The most troubling aspect is that the administration is insisting that its decisions in this area shouldn't be reviewed by any court, so no court has a role to play in setting the standards under which Americans are killed, in evaluating whether any particular American should be on a kill-list or the evidence itself."